The present invention relates to an image sensing apparatus and, more particularly, to an image sensing apparatus which obtains a high-resolution computer video signal by performing a so-called pixel displacement using optical path deflection means such as a parallel glass plate.
In recent years, video cameras are widely used as image input apparatuses or image sensing apparatuses for computers. In particular, a system as a combination of a video camera and a computer (e.g., a personal computer or workstation) is used as a DTP (Desktop publishing) system, a video electronic mailing system, a video meeting system, and the like.
Of these apparatuses, high-resolution image input apparatuses have been recently developed for use with an HDTV (High-definition television). Using such apparatuses, character and image data are edited and information is exchanged using high-definition images.
As an image input apparatus or an image sensing apparatus which can be used for such a system, an image sensing element having a large number of pixels is required to obtain a high resolution.
However, the number of pixels of the image sensing element of most existing video cameras is as small as about 250,000 to 400,000 pixels (although some elements have 580,000 pixels), and it is difficult to obtain a high-definition image. Such video cameras cannot be applied to the HDTV. Although high-resolution video cameras for a special purpose have been commercially available, they are not suitable for domestic equipment due to very expensive image sensing elements.
However, in recent years, a system which uses an image sensing element having about 400,000 pixels, and increases optical image information incident on the image sensing element by shifting the optical path by displacing the image sensing element or some lenses in the lens system so as to attain a high resolution has become commercially available. Owing to such a system, the price of an image input apparatus or an image sensing apparatus which can be applied to the HDTV is lowering.
The high-resolution system obtains a high resolution by a so-called pixel displacement using a parallel glass plate. The system will be briefly described below with reference to FIG. 1A.
Referring to FIG. 1A, reference numeral 201 denotes lenses for guiding an optical image from an object toward an image sensing element 202; 202, an image sensing element for converting the optical image into an electrical signal; 203, a holding frame which has rotation shafts 205 and 206 serving as the fulcrums of rotation at its two end portions in the horizontal direction, and holds a deflector 204; and 204, a parallel glass plate (deflector) which is fixed at the central portion of the holding frame 203. When the holding frame 203 rotates about the rotation shafts 205 and 206 as the center of rotation using a driving source (not shown), the parallel glass plate 204 disposed at the central portion of the holding frame 203 rotates (in the directions of an arrow a) in correspondence with the driving operation, thereby displacing incident light rays in the vertical direction.
A mechanism for a pixel displacement in the horizontal direction has a similar arrangement. More specifically, reference numeral 207 denotes a holding frame which has rotation shafts 209 and 210 serving as the fulcrums of rotation at its two end portions in the horizontal direction, and holds a deflector 208; and 208, a parallel glass plate (deflector) fixed at the central portion of the holding frame 207. When the holding frame 207 rotates about the rotation shafts 209 and 210 as the center of rotation using a driving source (not shown), the parallel glass plate 208 disposed at the central portion of the holding frame 207 rotates (in the directions of an arrow b) in correspondence with the driving operation, thereby displacing incident light rays in the horizontal direction.
FIG. 1B shows the relationship between the horizontal and vertical directions (X- and Y-directions) of the image sensing element 202, and a "Y-direction displacement effect" brought about by the rotation direction (the direction a) of the glass plate 203 and an "X-direction displacement effect brought about by the rotation direction (the direction b) of the glass plate 208. More specifically, when the glass plate 203 is rotated in the direction b, a pixel displacement is attained in the X-direction of the image pickup element 202; when the glass plate 208 is rotated in the direction a, a pixel displacement is attained in the Y-direction of the image pickup element 202.
Reference numeral 211 denotes an optical low-pass filter which changes the frequency characteristic of optical image information by utilizing the double refraction effect of a quartz. The low-pass filter 211 is normally constituted by at least two quartz plates. Of the frequencies of transmission light, one quartz plate of the low-pass filter 211 changes the frequency in the horizontal direction, and the other quartz plate in the vertical direction. The filter 211 is arranged in front of the image sensing element 202. Furthermore, the separation width of ordinary and extraordinary light rays generated by the double refraction effect of the quartz plates is appropriately set in advance in correspondence with the number of pixels and the arrangement of pixels of the image sensing element 202, a signal processing circuit, and the like.
A mechanism for displacing the optical path using the parallel glass plates 203 and 208 will be explained below with reference to FIGS. 2A and 2B. FIG. 2A shows a state wherein a parallel glass plate 221 is located parallel to the optical axis principal plane (in the plane), and FIG. 2B shows a state wherein the parallel glass plate 221 is displaced through an angle .theta. from the state shown in FIG. 2A.
Referring to FIGS. 2A and 2B, the glass plate 221 has a thickness d in the optical axis direction. Reference numeral 222 denotes incident light which becomes incident on the parallel glass plate 221; and 223, exit light which emerges from the parallel glass plate 221. The displacement amount, .delta., of the optical path by the parallel glass plate 221 is given by: ##EQU1## where N: the refractive index of the parallel glass plate
.phi.: the angle (incident angle) defined between the incident light and the plane normal PA1 .phi.': the angle defined between the incident light and the plane normal in the parallel glass plate PA1 cos .phi.=cos .phi.' PA1 sin .phi..apprxeq..phi.
If the incident angle .phi. is very small, since we have:
equation (1) above can be expressed by a simple approximation as follows: ##EQU2##
Thus, if the displacement amount of the optical path in FIG. 2A is .delta.=.delta..sub.1, and the displacement amount of the optical path in FIG. 2B is .delta.=.delta..sub.2, the following relationships are established: ##EQU3## When the parallel glass plate is inclined through .theta. from the state shown in FIG. 2A (i.e., the state shown in FIG. 2B), the optical path change amount .delta..sub.s is: ##EQU4## More specifically, when the parallel glass plate 221 is rotated through .theta., the optical path changes by .delta..sub.s in the principal plane. The above-mentioned principle of changing the optical path shown in FIGS. 2A and 2B applies to the glass plates 203 and 208 shown in FIG. 1A although their rotation directions are perpendicular to each other.
An example of the pixel arrangement and aperture of the image sensing element 202 will be briefly described below with reference to FIGS. 3A and 3B.
Referring to FIG. 3A, reference symbol H denotes a horizontal scanning direction for the image sensing element 202; and V, a vertical scanning direction. On one of two adjacent horizontal lines, yellow filters Y and magenta filters M are alternately disposed at a pixel interval p.sub.h in the horizontal scanning direction, and on the next line, cyan filters C and green filters G are also alternately disposed at the pixel interval p.sub.h. These lines are alternately disposed at a pixel interval p.sub.v in the vertical scanning direction.
If the thickness d of each parallel glass plate is set so that the displacement amount of the optical path becomes 1/2 the pixel size, i.e., 1/2.multidot.p.sub.h and 1/2.multidot.p.sub.v when the above-mentioned parallel glass plates 203 and 208 are inclined through the angle .theta., a x16 image information amount can be obtained by changing the optical path four times in the horizontal direction and four times in the vertical direction, as shown in FIG. 3B, thus attaining a high resolution using a conventional image sensing element with a small number of pixels.
In the image sensing apparatus shown in FIG. 1A, the mechanism for the pixel displacement in the horizontal direction is completely separated from that for the pixel displacement in the vertical direction, and the two parallel glass plates 203 and 208 can be operated independently.
However, in the conventional arrangement shown in FIG. 1A, since the two independent parallel glass plates are juxtaposed in the optical axis direction, the total lens length increases due to an increase in thickness in the optical axis direction. Also, the distance from the lens rear end portion to the image sensing element, i.e., the back focus is prolonged, and it is difficult to obtain a desired optical characteristic. Furthermore, since the parallel glass plates 203 and 208 must be independently arranged in the horizontal and vertical directions, the number of parts and cost increase.
FIG. 4 shows the arrangement of a conventional image sensing apparatus which reduces the number of parallel glass plates to one so as to solve the problem of FIG. 1A.
The image sensing apparatus system shown in FIG. 4 comprises lenses 201, a filter 211, and an image sensing element 202 as in the image sensing apparatus shown in FIG. 1A. Note that FIG. 4 is a schematic view showing horizontal and vertical driving portions of a parallel glass plate of the image sensing apparatus when the optical axis principal plane is viewed from the lens side.
Referring to FIG. 4, reference numeral 231 denotes a parallel glass plate; and 232, a frame for holding the parallel glass plate 231. Reference numerals 233 and 234 denote rotation shafts arranged at the two end portions, in the horizontal direction, of the frame 232. These rotation shafts 233 and 234 support the frame 232 (i.e., the glass plate 231) to be rotatable about an axis A (consequently, in the vertical direction of the image sensing element 202) with respect to a frame 237 (to be described below). Reference numeral 235 denotes a cam pin arranged on the lower end of the frame 232. A portion of the cam pin 235 contacts the cam surface of a cam 236. When the pin 235 moves vertically, the frame 232 is rotated about the axis A. The cam 236 is rotated by a stepping motor 247, and has a substantially spiral shape so that its radius changes in correspondence with the rotation angle. The cam 236 vertically moves the cam pin 235 by its rotation, thereby rotating the frame 237 about the axis A.
FIG. 5 is a view showing the cam 236 when viewed from the front side of the rotation shaft of the stepping motor 247. As shown in FIG. 5, the cam 236 has a plurality of segments. Since these segments have different radii by a predetermined amount, the cam pin 235, which is in a contact with the outer circumferential surface of the cam 236, is displaced in the vertical direction in FIG. 5 upon rotation of the stepping motor 247.
Reference numeral 237 denotes a frame which supports the frame 232 via the rotation shafts 233 and 234 to be rotatable about the axis A. The frame 237 has an opening, and hence, has a substantially rectangular shape. Bearing portions 238 and 239 which rotatably engage with the rotation shafts 233 and 234 are arranged at the two end portions, in the horizontal direction, of the opening. Rotation shafts 241 and 242 (the shaft 242 is not seen since it is located below the cam pin 235 in FIG. 4) are arranged on the two end portions, in the vertical direction, of the outer portion of the frame 237, and the frame 237 is held by bearing portions 243 and 244 (which is located below the cam pin 235 in FIG. 4) arranged on the two end portions, in the vertical direction of a base 250 so as to be rotatable about an axis B in the horizontal direction with respect to the base 250.
Reference numeral 240 denotes a cam pin arranged on one end of the frame 237. A portion of the cam pin 240 contacts the cam surface of a cam 249. When the pin 240 vertically moves in a direction perpendicular to the plane of the drawing of FIG. 4, the frame 237 is rotated about the axis B. The cam 249 is rotated by a stepping motor 248, and has a substantially spiral shape, so that its radius changes in correspondence with the rotation angle. The shape of the cam 249 is substantially the same as that of the cam 236 shown in FIG. 5. When the cam pin 240 is vertically moved upon rotation of the cam 249, the frame 237 is rotated about the axis B.
Reference numeral 245 denotes a coil spring wound around the rotation shaft 241 of the frame 237; and 246, a coil spring wound around the rotation shaft 234 of the frame 232. The springs 245 and 246 normally bias the cam pins 250 and 235 against the cams 249 and 236, respectively. Note that the stepping motors 247 and 248 are fixed to the base 250.
In the above arrangement, when the stepping motors 248 and 247 are driven, the cams 249 and 236 rotate, and the cam pins 240 and 235 which are in a contact with the cams move in the direction perpendicular to the plane of the drawing in FIG. 4, thereby displacing the parallel glass plate 231 by a very small amount in the horizontal and vertical directions. The pixel displacement can be attained by this displacement, and substantially the same effect as that obtained when an image sensing element with a large number of pixels is used can be obtained.
According to the above-mentioned arrangement shown in FIG. 4, since the axis B extending through the rotation shafts 241 and 242 used for rotating the frame 237, and the axis, in the longitudinal direction, of the cam pin 235 for moving the frame 232 vertically in the plane of the drawing of FIG. 4 are located on a single line, the operation for rotating the frame 232 about the axis A, and the operation for rotating the frame 237 about the axis B are completely independent from each other. More specifically, although the frame 237 for supporting the rotation shafts 238 and 239 of the holding frame 232 is not fixed but rotates about the rotation shaft 241, the cam pin 235 for giving a rotation moment to the frame 232 matches the axis B, and the rotation of the frame 237 will not influence the frame 232.
However, according to the prior art shown in FIG. 4, since a driving portion for rotating the frame 237 about the axis B (i.e., a horizontal pixel displacement driving portion) and a driving portion for rotating the frame 232 about the axis A (i.e., a vertical pixel displacement driving portion) are arranged in orthogonal directions, driving motors cannot be appropriately arranged, and hence, the thickness of a lens unit portion must be increased. As a result, the entire apparatus becomes large in size.